After reading the novel Know No Fear, I've heard of the Ultramarines' great domain before the time of the Heresy. Within the story it spoke of the XIII Legion's great realm of five hundred worlds.
Known as the Five Hundred Worlds of Ultramar, or "the New Empire", the realm was the home of the largest of the Space Marine Legions. Mainstream 40k knowledge doesn't have much detail on this realm, so I took it upon myself to name all 500 worlds.
This project tok me two months to accomplish. I surfed the Web, searching for suitable names. Of course, I included the original eleven worlds of Ultramar, as well as any future homeworlds of Ultramarines Successor Chapters, and a few other Imperial worlds. The rest of the worlds are named after actual ancient locations on Earth, mostly from Ancient Greece, Rome, Mesopotamia, or my own derivatives. For example, I may have taken a few of our modern cities' names and re-spelled them, or found ancient-sounding names, especially of Roman origin.
The end result, a list of my own idea of Ultramar: 500 worlds exactly.